The present invention relates generally to the field of circuit elements that are made with thick film fabrication techniques. More specifically, the invention relates to a thick film switching element that is well-adapted for use as an encoder contact element. The invention also relates to a method of fabricating such an element.
Switching elements in the form of encoder contact elements are commonplace in many electrical and electronic products. A typical prior art encoder contact element comprises a pattern of conductive areas or contacts printed, etched, or deposited on an insulative substrate, usually along a circular or arcuate track, whereby the contacts are sequentially contacted by a conductive wiper so as to produce an encoded electrical signal. For example, the track may comprise alternating conductive contacts and nonconductive areas so as to produce a square wave as the wiper is moved along the track.
There have been many techniques and materials used in the past to fabricate switching elements that are configured for use as encoder contact elements. One technique that has been used is the deposition or printing of thick film conductive inks, such as cermet inks, to form the contacts. This construction produces contacts that are elevated above the surface of the insulative substrate. Consequently, as the wiper passes repeatedly along the encoder track, both the wiper and the contacts are subject to relatively rapid wear, decreasing the lifetime (in terms of rotational cycles) of the encoder. Thus, where durability (long lifetime or a large number of rotational cycles) is desired, thick film cermet inks may not meet performance criteria.
Another technique is described in U.S. Pat. No. 5,702,653-Riley. In the Riley '653 patent, the contacts are formed by fusing a high-temperature glass frit to a non-conductive substrate. A cermet layer having a low temperature glass matrix is applied in the desired encoder pattern to the surface of the frit, and then fired so as to sink into the frit. This results in the cermet forming contacts that are sunk into the frit such that the thickness of the contacts is approximately equal to the original thickness of the frit layer. In actual practice, as described in the '653 patent, the contacts extend above the surface of the frit layer by a distance of about 4 to 10 microns. A similar technique is disclosed in U.S. Pat. No. 5,169,465-Riley.
While the fabrication methods of the aforementioned patents (U.S. Pat. No. 5,702,653 and U.S. Pat. No. 5,169,465) provide improved wear characteristics over conventional thick film fabrication methods, they are limited to encoder devices employing substrate materials that can withstand the relatively high firing temperatures involved (i.e., above the melting point of the glass frit). Thus, while suitable for ceramic and metal substrates, these techniques cannot be used to make encoders that include substrates formed from typical printed circuit ("PC") board materials, such as fiberglass-filled polymeric resins.
Thus, there has been an heretofore unmet need for an encoder element formed on a PC board substrate that provides good wear-resistance with a low contact profile. There has also been a need for method of fabricating such an element.